Identification of genetic mutations linked to familial neurodegenerative diseases have made it possible to generate useful transgenic animal models. Studies using these transgenic animals indicate that many familial neurodegenerative diseases, such as motor neuron disease, Alzheimer's disease, prion diseases and trinucleotide repeat diseases, result from a gain of deleterious properties. The disease-specific pathology in transgenic mice demonstrates the utility of these models in elucidating pathogenic mechanisms of the disease and in developing therapeutic strategies.